The present invention relates to a semiconductor memory device and a method of producing the same and, more specifically, relates to a nonvolatile memory device, for example, a flash EEPROM (flash electrically erasable programmable read-only memory), in which an insulating film containing metal or semiconductor particles is used in a gate of a transistor.
Conventionally, in an MOS-type transistor used to store information, a semiconductor memory device has been proposed in which a portion containing conductive silicon clusters in insulating silicon is disposed above the channel area but below the control gate electrode.
One such conventional semiconductor memory device, shown in FIG. 6, comprises an n-channel transistor, a source region 13 and a drain region 12 of n-type semiconductor material, an Si substrate 11 which is a p-type semiconductor, and a gate region including a polysilicon layer 16 and an insulating region having two oxide layers 14 and a nitride layer 15 disposed between the oxide layers 14. When electrons are injected into the insulating region adjacent to the channel layer in the substrate during operation, this raises the gate threshold voltage which is necessary to render the channel layer conductive and, when electrons are withdrawn from semiconductive silicon clusters, the gate threshold voltage is reduced. In this way, the device can be arranged to retain data by applying a gate sensing voltage which is between the threshold values in the initial state and in the electron-injected state so as to give an on-off control to the transistor.
This operation is specifically described in the article entitled "A True Single-Transistor Oxide-Nitride-Oxide EEPROM Device" (IEEE Electron Device Letters, Vol. EDL-8, No. 4, March 1987). In this operation, data writing is normally carried out by electron injection, while data erasing is carried out by hole injection. After 40,000 write/erase cycles, the erased state and storage-state characteristics are degraded. In order to eliminate such drawbacks, this article proposes that the stored charges representing data be withdrawn into the drain region by tunneling. However, it is difficult for electrons to flow in a lateral direction in the nitride layer.
In conventional data storage devices, each memory cell must be made of only one transistor (1Tr/cell) to provide high-density integration. However, the method of integration of such devices has not been resolved and the removal of electrons from the drain region and the injection of electrons into the drain region have not yet been accomplished effectively.